The video solution is finally available, we'll see how many did it
the way I did.

So if you, or anyone can tell me what I'm doing wrong please?Stand on yellow to let the lower cannon shoot one of the two boxes near darkblue button. then push a reflector in front of that cannon to make sure it doesn't shoot the other box. Put a box on yellow, push box on dark blue and get out of there. the last box on the bottom right white button to take the bonus coins on the other buttons.
At that moment I have only one reflector which I can move. I push the lower cannon up and put the reflector in line of the upper cannon and the lower of the three prisms. When the cannon has shot I quickly put the reflector on the yellow button to catch the returning beam. That moment the red dude has gone but I've nothing left to move, so I can't get the coin.
At least, that's the way I did.The cannonfire to kill the dude should go from the lower cannon > reflector > lower - middle - upper prism > other reflector on yellow (lower cannon should be pushed up) > prism below upper cannon > reflector top left > red dude.
But if that's not enough, I will write you a walktrough if no one else does.One should be in line with the lower cannon and the lower of 3 prisms. The other in line with the upper cannon and the upper prism so that's on the yellow button. You have to make the way free for the cannonfire, so the upper and bottom bonuscoin have to be collected and the lower cannon should be pushed up after it shoots.
"Continued..........Well a big DUH for me...........
Finally got it....."